The present invention relates to a film forming system having a pair of opposed electrodes to which high frequency power supply voltages are supplied. The present invention particularly but not exclusively relates to a high frequency bias sputtering system in which high frequency power supply voltages are applied to a substrate electrode and a target electrode for performing high frequency bias sputtering.
A typical example of a conventional high frequency or radio frequency (hereinafter referred to as RF) bias sputtering system is disclosed in Japanese Patent (unexamined) Laid-open Publication 63(1988)-18071, in which RF power is applied as target bias to a target electrode from an RF power supply and RF power is also applied as substrate bias to a substrate electrode from the power supply voltages supply. The RF powers must be simultaneously applied to the electrodes in bias sputtering.
When the RF power is applied to the target electrode in such a simultaneous manner, an RF discharge plasma is formed between the target electrode and the substrate electrode. In the plasma, more ions gather at the target than at the substrate, because of a difference between mobility of ions and electrons, so that a cathode dark space is formed in the vicinity of the target. Thus, negative bias is induced on the surface of the target, which is hence bombarded by ions due to the attractive force of the electric field, thereby sputtering the target material. When the RF power is supplied to the substrate electrode, the surface of the substrate is also bombarded by ions, as is the target, and thus reverse sputtering of the substrate occurs.
In the bias sputtering, adjustment of the power supply voltages of the electrodes enables the plasma confinement to be enhanced in efficiency and a flat film to be formed over an irregular surface of the substrate.
It is empirically known that when the RF power supply voltage applied to the electrodes are maintained within a predetermined range of phase difference, desired effects are produced for stabilization of discharge and for improving efficiency. An attempt which has taken this finding into consideration is reported in IBM Technical Disclosure Bulletin, Vol. 14, 1032 (1971), in which power is supplied to a substrate electrode and a target electrode from the same RF power supply by RF power supply cables, and an appropriate cable length is selected for one of the RF power supply cables.
In this earlier attempt, the RF power supply voltages which are applied to the electrodes are liable to be shifted in phase due to variations of film forming parameters at the start of the sputtering and due to changes of the parameters with the lapse of time even if the substrate electrode and the target electrode are each supplied with RF power from the same power source on the condition that a correct phase relationship is established by adjusting the cable length as described. This results in that the discharge becomes unstable.
Recently, high has become required for thin films as thin film devices become highly functional. Thus, such an instability of discharge largely affects properties of the thin films formed by the technique and should be suppressed as much as possible. For this purpose, it is essential to control discharge parameters to predetermined values in addition to the other values to be controlled.
Another attempt is disclosed in Japanese Patent (unexamined) Laid-open No. 59 (1984)-205477, in which a uniform thin film is formed over a substrate by applying RF power supply voltages, having different phases, to a target electrode and a substrate electrode of an RF bias sputtering system.
In this RF bias sputtering system, the RF output of the RF power supply unit is split to apply it to the target electrode and the substrate electrode through a phase adjusting device. The RF bias sputtering system has a phase detecting circuit arranged in each power supply cable connected to the corresponding electrode. Output signals of the phase detecting circuits are inputted to a phase difference detector, where a voltage proportional to the phase difference between the RF powers is produced according to the signals. Thus, the sputtering is performed by keeping the phase difference between the RF voltages of the electrodes at a predetermined value by the phase adjusting device according to the produced voltage.
This RF bias sputtering system is superior to the other conventional sputtering systems in that the phase difference between RF voltages of the electrodes is constantly maintained. However, the phase detecting circuits which are provided in the power supply cables are influenced by inductance and capacitance of the cables, and hence it is not possible to exactly detect a change in phase due to a variation in the state of discharge. Thus, the RF bias sputtering system is inferior to the other known art in accuracy of the voltage phase control of the applied powers, and hence it is hard to stabilize the discharge. The known RF bias sputtering system this is insufficient to form a thin film of high performance.
Moreover, in the known RF bias sputtering system the phase adjusting device adjusts phases of the RF power supply voltages sent to the power supply cables, and hence the phase adjusting device must withstand high voltages. This results in that the phase adjusting device becomes large sized and costly.